Abstract—A low phase noise and high power MMIC VCO

using GaN-on-Si is presented. The VCO is based on common

source series feedback to generate the negative resistance,

using 0.35 m GaN HEMT on silicon substrate technology. The

VCO can be tuned, between 3.92 GHz and 4.39 GHz, and has

low phase noise, of-119 dBc/Hz, at a 1 MHz offset. The output

power of the VCO is 14.5 dBm at 4.2 GHz from a 20 V power

supply, while the total die size was 1.35 mm2.

Index Terms—VCO, output power, phase noise, GaN HEMT,

GaN-on-Si.

I. INTRODUCTION

Gallium Nitride (GaN) devices are of great interest

because of their suitability for high power applications.

AlGaN/GaN High Electron-Mobility Transistor (HEMT)

technology has established itself as a strong contender for

such applications, because of its large electron velocity

(>1×107 cm/s), bandgap (3.4 eV), breakdown voltage (>100

V) and sheet carrier concentration (ns > 1×1013 cm-2).

Most AlGaN-GaN HEMTs have been grown on sapphire

[1]-[2], or SiC substrates [3]-[13]. The sapphire substrates

are low-cost, but inefficient in heat dissipation, due to their

poor thermal conductivity. GaN HEMTs on semi-insulating

SiC have excellent crystal quality and thermal dissipation,

due to reduced lattice mismatch and high thermal

conductivity (4.9 W/cm∙K). The disadvantages of GaN-on-

SiC are its higher cost and unstable crystal quality.

Nowadays, there is increasing interest in growing AlGaN-

GaN HEMT structure on Si substrate [14]-[20], which has

the advantages of low cost, large-size substrate and good

thermal conductivity (1.5W/cm∙K). The main challenge of

GaN-on-Si is the cracking of GaN film due to stress.

Recently, several authors presented studies on the power of

AlGaN-GaN HEMTs on silicon substrate, at different

frequencies [15]-[20]. The results, which demonstrate the

capability to produce high power amplifiers and oscillators,

make AlGaN-GaN HEMTs very attractive for future

communication systems. The VCOs are also indispensable

in fully integrated AlGaN/GaN HEMT transceivers. For

GaN oscillators, some reports have presented the oscillators

using GaN-on-SiC or GaN-on-sapphire in hybrid systems

[1], [3]-[5], [14] or in monolithic microwave integrated

circuit (MMIC) [6]-[13]. Only one has been reported a GaN-

Manuscript received January 20, 2013; revised May 9, 2013. This work

was supported in part by NSC 100-2221-E-182-034, 99-2632-E-182-001-MY3, CGURP UERPD 2A0011 and NDL 99-C03S-046 of Taiwan.

The authors are with Dept. of Electronic Engineering, Chang Gung Univ., Tao-Yuan, Taiwan (e-mail: snoopy@mail.cgu.edu.tw).

on-Si power oscillator using hybrid system [14]. We

proposed a 4.2 GHz 0.35 m GaN HEMT VCO based on the

negative resistance concept using a common-source series

feedback element. The VCO exhibits a frequency tuning

range from 3.92 to 4.39 GHz with the varactor’s voltage

from -8 to 4 V. The phase noise of-119 dBc/Hz at 1MHz

frequency offset at 4.2 GHz and output power of 14.5 dBm

at a gate bias of -2.5 V and a drain bias of 20 V is achieved.

The chip size is 1.5 × 0.9 mm2.

a)

b)

Fig. 1. The a) structure and b) SEM photograph of GaN HEMT.

II. DEVICE EPI-STRUCTURE AND PROCESS

Fig. 1 shows the schematic of GaN HEMTs structure. The

devices used in this work were grown on silicon (111)

substrate, using molecular beam epitaxy. The resistivity of

the Si substrate was about 104 Ω•cm. The epi-layer

contained an AlN/GaN nucleation layer, a 1.8-um-thick

layer of unintentionally doped GaN buffer, a 1-nm-thick

AlN space layer, a 18-nm-thick Al0.27Ga0.73N barrier and a

1-nm-thick unintentionally doped GaN cap layer. Hall

measurements confirmed a sheet-carrier density of

1.031013

cm-2

and an electron mobility of 1,534 cm2/V-s.

Following mesa etching, ohmic contacts were prepared,

using evaporated Ti-Al-Ni-Au multilayer metals, followed

High Power and Low Phase Noise MMIC VCO Using

0.35m GaN-on-Si HEMT

Hsuan-ling Kao, Bo-Wen Wang, Chih-Sheng Yeh, Cheng-Lin Cho, Bai-Hong Wei,

and Hsien-Chin Chiu

International Journal of Computer Theory and Engineering, Vol. 5, No. 6, December 2013

910DOI: 10.7763/IJCTE.2013.V5.821

by annealing. A mushroom-shaped gate, based on Ni-Au

metallization, is defined by a tri-layer resist scheme. The

surface was passivated with SiO2. The thickness of the

silicon substrate was reduced to 100 m, for better DC and

RF power performance. All the HEMTs have a gate length

of 0.35 m. The HEMT devices display a maximum drain

current density of 576 mA/mm and dc extrinsic

transconductance (gm) of 150 mS/mm. The unity current

gain cutoff frequency (ft) and maximum oscillation

frequency (fmax) of the devices are 14.9 and 46.6 GHz. The

maximum output power density of at 3.5 GHz is 1.58

W/mm (Vds=10 V).

Fig. 2. Schematic of the ADS-designed 4.2 GHz VCO circuit which uses

0.35 m GaN HEMT on Si-substrate technology.

-5 -4 -3 -2 -10.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

10

15

20

25

30

35

40

Ca

pa

cit

an

ce

(p

F)

Vg (V)

Qu

ali

ty F

ac

tor

width=2x100 m

Fig. 3. The capacitance and Q-factor of varactors(Cv1).

III. DEVICE EPI-STRUCTURE AND PROCESS

This circuit uses a 0.35 m GaN HEMT on Si-substrate

process, provided by the foundry. The MMIC uses both

airbridge to form a global interconnect layer and slot via

under every source to achieve high gain and excellent

thermal properties for circuit design. Fig. 2 shows the design

of the 4.2 GHz VCO. The design is based on the negative

resistance concept using a common-source series feedback

topology. The VCO circuit is composed of M1, Cv1, C1~C3,

Ld, Lg, Ls and L1~L3. Cv1 is the varactor of 2 100 m width

HEMT devices with the source-drain tied together as a two

port network. The capacitance and Q-factor versus voltage

are shown in Fig. 3. The operation bias of the varactor

simulation is consistent with the VCO circuit in Fig. 2. The

minimum capacitance is 0.22 pF, while the maximum

capacitance is 0.69 pF. The width of M1 is also 2100 m.

Ls was connected between the M1 source and ground,

providing a positive feedback to make the transistor M1

unstable. A shunt capacitor C1 was in parallel with the Ls to

shorten the stub’s length and compact chip size. The LC

tank, including L1~L3, Lg, C3, Cv1, was connected to the gate

of the M1. C3 is used also decoupled the negative gate bias

and varactor’s control voltage (Vcontrol). C2 is DC block

capacitors. Ld is the DC feed inductor. GaN oscillators have

large output signals without gain stage between core and

load.

IV. DEVICE EPI-STRUCTURE AND PROCESS

The common-source series feedback VCO was simulated,

using Advance Design System (ADS) software. Fig. 4

shows the layout of the fabricated VCO. Its size is 1.5 0.9

mm2, including the probe pads. The 4.2 GHz VCO was

tested on a wafer – the spectral density of the circuit being

measured with a spectrum analyzer. The circuit is biased at

Vds=20V, Ids=38 mA, and Vgs=-1.8V. The output spectrum of

the proposed VCO is shown in Fig. 5. The center frequency

is 4.2 GHz and output power is 14.5 dBm, including a 2.6

dB loss due to implementation. The rf-to-dc efficiency of

the 4.2 GHz VCO is 3.8 %.

Fig. 4. Chip Image of a 4.2 GHz VCO circuit which uses 0.35m GaN

HEMT on Si-substrate technology.

Fig. 5. The output power of a 4.2 GHz VCO circuit.

The output power of a 4.2 GHz VCO circuit which uses

0.35 m Fig. 6 shows the phase noise, at 1 MHz offset,

when operating at a controlled voltage of -4 V. The lowest

phase noise is about-119 dBc/Hz, at 1 MHz offset from the

4.2 GHz carrier frequency. According to the Leeson formula

[21]:

)}(P

FkT])([log{)(L

m

c

sL

mf

f1

2Q

f

f

11

2

110f 20

2

m

(1)

where L(fm) is single side band phase noise density, fm is the

International Journal of Computer Theory and Engineering, Vol. 5, No. 6, December 2013

911

offset frequency, QL is the loaded quality factor, fo is the

resonant frequency, Ps is the amplifier’s input signal power,

F is its noise figure, k is the Boltzman constant, T is the

temperature, fc is the flicker corner frequency of the device.

It can be seen phase noise is inversely proportional to

output power, so a large power is favorable from this point

of view. Despite this, the demonstrated GaN-on-Si oscillator

shows better phase noise compared to GaN-on-sapphire [2]

or GaN-on-SiC [6]-[13].

Fig. 6. The phase noise of a 4.2 GHz VCO circuit.

-10 -8 -6 -4 -2 0 2 4 63.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

Fre

qu

en

cy

(G

Hz)

Vcontrol

(V)

Fig. 7. The tuning range of a 4.2 GHz VCO circuit.

-8 -6 -4 -2 0 2 411

12

13

14

15

Ou

tpu

t P

ow

er

(dB

m)

Vcontrol

(V)

Fig. 8. The output power of a 4.2 GHz VCO0 circuit.

Fig. 7 shows the measured oscillation frequency of the

VCO versus the controlled voltage (Vcontrol) applied on the

varactor. The proposed common-source series feedback

VCO was tuned, from 3.92 GHz to 4.39 GHz, which is a

tuning range of 468 MHz (near 11.4 % tuning range), with a

tuning voltage varying from -8 to 4 V. The output power of

the VCOs as a function of control voltage is shown in Fig. 8.

Measured output power is 12.9 ± 1.7 dBm on a large

frequency tuning range of 468 MHz. The maximum output

power is 14.5 dBm at 4.2 GHz.

Table I summarizes the measured performance of the

VCO and includes other reported performances, for the

purpose of comparison. Our differential VCO using GaN-

on-Si technology achieved low phase noise and high output

power, comparing well with, or better than, other published

figures [6], [13], [22], [23].

TABLE I: SUMMARY OF MMIC OSCILLATOR FROM PREVIOUS STUDIES

AND THAT OF THIS STUDY

Ref. [6] [13] [22] [23] This Work

Process GaN

HEMT on SiC

GaN

HEMT on SiC

CMOS CMOS HEMT on Si

fo (GHz) 5 4.166 5.63 5 4.2

Tuning

Range single

33 kHz (0.7 %)

2.6GHz (45%)

390MHz (8.3%)

468 MHz (11.4 %)

Phase Noise

(dBc/Hz)

-132@

1MHz

-116@

1MHz

-109@

1MHz

-120@

1MHz -119@ 1MHz

Pout (dBm) 28.4 ~

32.8 17.6~22.9 -9.88 2.2 14.5

Efficiency

(%) 11~21.5 8.8 0.7 28 3.8

V. CONCLUSIONS

The fully integrated GaN HEMT VCO on Si-substrate

demonstrated good circuit performance, in terms of high

output power and low phase noise. The circuit was

fabricated, using a 0.35 m GaN HEMT process. This 4.2

GHz VCO displayed a phase noise of-119 dBc /Hz, at a 1

MHz offset, and a 14.5 dBm output power while the total

die size was 1.35 mm2.

ACKNOWLEDGMENT

The authors wish to thank CIC of the National Science

Council and HSIC Research Center of CGU in Taiwan for

their help. This work was partially supported by NSC 100-

2221-E-182-034, 99-2632-E-182-001-MY3, CGURP

UERPD2A0011 and NDL 99-C03S-046 of Taiwan.

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Hsuan-Ling Kao received the Ph.D. from Department of Electronics

Engineering, National Chiao Tung University, Hsinchu, Taiwan, in 2006.

She was with Macronix (MXIC) from 2000 to 2006. In Oct. 2006, she joined Electronics Engineering Department of Chang Gung University and

focused on microwave, millimeter wave integrated circuits and devices.

Bo-Wen Wang received the B.S. degree in electrical engineering from

Chung Yuan Christian University, Taoyuan, Taiwan, in 2011. He is currently working toward the M.S. degree at the Electronics Engineering

Department of Chang Gung University.

Chih-Sheng Yeh received the B.S. degree in electrical engineering from Fu

Jen Catholic University, Taipei, Taiwan, in 2010. He is currently working toward the Ph.D. degree at the Electronics Engineering Department of

Chang Gung University.

Cheng-Lin Cho received the B.S. degree in electrical engineering from

National Formosa University, Yunlin County, Taiwan. He is currently working toward the M.S. degree at the Electronics Engineering Department

of Chang Gung University.

Bai-Hong Wei received the B.S. degree in electrical engineering from

Chang Gung University, Taoyuan, Taiwan. He is currently working toward the M.S. degree at the Electronics Engineering Department of Chang Gung

University.

Hsien-Chin Chiu was born in Taipei, Taiwan, R. O. C. He received the B.

S. degree and the Ph.D. degree in electrical engineering from National

Central University, Chungli, Taiwan, in June, 1998 and January, 2003,

respectively. He joined the Department of Electronic Engineering, Chang

Gung University in 2004, and now he is a professor of this university. His research interests include the microwave, millimeter wave integrated

circuits, GaAs and GaN FETs fabrication and modeling.

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